1. Field of the Invention
The present invention relates to a producing method of a CMOS image sensor.
2. Description of the Related Art
Conventionally, in a CMOS image sensor, the surface e layer of its photodiode converting light into an electric signal has been formed of only an oxide film. The incident light passes through the oxide film having a refractive index of approximately 1.46 from air having a refractive index of approximately 1, and impinges on the photodiode formed from silicon having a refractive index of about 3-5. When light thus impinges on a substance having a higher refractive index from a substance having a lower refractive index, the incidence efficiency of the incident light on the photodiode reduces because of the reflection caused at the interface therebetween. The greater the difference between their refractive indices, the more reflection is caused at the interface. For this reason, Non-patent Literature 1 describes the device in which a layer having an intermediate refractive index between the ones of the oxide film and the silicon forming the photodiode is formed therebetween, and the incidence efficiency of the incident light on the photodiode is thereby improved by reducing the reflection (Refer to Non-patent Literature 1).
The multilayer antireflection film described in the Non-patent Literature 1 is formed in a three-layered structure of an oxide film, a nitride film, and an oxide film. The transmittance of the incident light on the photodiode is improved by forming a nitride film over the photodiode through the medium of a thin oxide film in the literature. However, such an arrangement needs the process of forming the nitride film over the photodiode as an antireflection film, and moreover such an antireflection film has a limitation in control of the dependence of the transmission characteristic on the wavelength. That is, such an antireflection film only improves the light transmittance in the visible region, but the film cannot reduce the transmittance of near infrared rays deteriorating the color reproduction of the color image sensor.
On the other hand, Patent Literature 1 discloses the following technique serving as a producing method of a conventional CMOS image sensor. A mask pattern exposing only the portion that becomes a low-concentration diffused region within the P type well, is formed by photolithography. The exposed area is doped with low-concentration N type impurities by means of ion implantation of phosphorus or the like, to thereby form a low-concentration diffused region therein. A mask pattern covered with a photodiode and silicon oxide film is formed on the P type well by photolithography, and both of the side areas of the gate electrode are doped with N type impurities by means of ion implantation, to thereby form a high-concentration diffused source region and a high-concentration diffused drain region in the P type well. At that time, because the gate electrode and its sidewalls serve as a mask, low-concentration diffused regions remain directly underneath the zone, and thereby a MOSFET having a so-called LDD structure is formed.
Further, a multilayer antireflection film is formed by alternately depositing an insulating film formed of oxide film and an insulating film formed of nitride film over the region including the surfaces of the photodiode and the MOSFET. A predetermined mask pattern is formed over the multilayer antireflection film by photolithography. The multilayer antireflection film is left only over the surface of the P type diffused layer that becomes the photodetecting surface of the photodiode and its adjacent region, and the antireflection film locating over the other regions is removed by etching.
Subsequently, a first interlayer insulation film is formed over the region including the surfaces of the multilayer antireflection film, the diffused layers, and the gate electrode, then contact holes electrically connecting the diffused layers and wiring described later are formed within the interlayer insulation film by the predetermined etching process, and after that contact plugs are inserted in the contact holes. In addition, a second interlayer insulation film is deposited over the first interlayer insulation film such that the second interlayer insulation film is connected with these contact plugs, and then a light-blocking film is formed by sputtering procedure or the like over the surface of the second interlayer insulation film other than the portion that becomes the photodetecting surface of the photodiode, to thereby produce a CMOS image sensor. The conventional CMOS image sensor has been produced in this manner.
Non-Patent Literature 1:
“High-Sensitivity and No-Crosstalk Pixel Technology for Embedded CMOS Image Sensor,” IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 10, October 2001, pp. 2221-2227.
Patent Literature 1:
Japanese Kokai Patent Publication No. 2002-083949/2002, pp. 3-5, FIGS. 1-9.
Because the conventional producing method of the CMOS image sensor is carried out as mentioned above, there have been the problem that the process of separately forming the nitride film serving as the antireflection film over the photodiode is needed, and moreover the antireflection film has a limitation in control of the dependence of the transmission characteristic on the wavelength. That is, there has been the problem that the conventional producing method of the CMOS image sensor needs a large number of processes. Furthermore there has been the problem that the antireflection film produced by the conventional producing method of the CMOS image sensor only improves the light transmittance in the visible region, but the film cannot reduce the transmittance of near-infrared rays deteriorating the color reproduction of the color image sensor.